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Cancer-Deterring Drug Found to Harm Bones

A drug that scientists hoped might be a safe new way to prevent breast cancer appears to pose a risk after all: significant bone loss.

The finding, published online Monday in The Lancet Oncology, could make women more reluctant to use the drug, exemestane, and it deals a setback to the notion that one day healthy people might take medicine to reduce their risk of getting cancer.

“One might not be too reassured about the use of exemestane in the prevention setting,” Jane A. Cauley, an epidemiologist at the University of Pittsburgh, wrote in a commentary that accompanied the study.

Exemestane (brand name Aromasin) is already used to prevent recurrence of breast cancer. But a large study published last June showed its use could reduce the risk of getting invasive breast cancer in the first place by about 65 percent, compared with a placebo, in women at higher risk of the disease.

While two other drugs, tamoxifen and raloxifene, are already approved to prevent breast cancer, they are rarely used for that purpose, in part because of serious side effects like blood clots. But exemestane does not have those side effects, leading researchers to hope that the drug could become a viable option for millions of women.

Dr. Angela M. Cheung, the lead investigator of the new study, said its findings should not discourage women at high risk of getting breast cancer from considering the drug.

“Sometimes the options are, ‘Should I take my breasts out, or should I take a medication such as this?’ ” said Dr. Cheung, who is director of the osteoporosis program at University Health Network in Toronto.

The new study is actually a more detailed evaluation of bone quality in 351 of the 4,560 postmenopausal women participating in the study published in June. The women were evaluated by a relatively new technique called high-resolution peripheral quantitative CT. After about two years, the researchers found, those who were taking exemestane had an average 6.1 percent decline in the bone mineral density in the wrist, compared with 1.8 percent in decline for women getting a placebo.

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The exemestane users also had more evidence of a weakening of bone structure. That suggested that conventional bone density tests may not be able to detect all the damage caused by the drug, the researchers said.

The study was too small and too short to detect any difference in the number of fractures.

Exemestane is one of a class of drugs called aromatase inhibitors, which lower the body’s levels of estrogen, a hormone that can fuel breast cancer growth. Since estrogen is important for bone health, it was long suspected that exemestane might be bad for bones. But this was the largest study comparing it against a placebo in healthy women, Dr. Cheung said.

Dr. Paul E. Goss, the lead investigator of the overall study published in June, said the new findings contained “no new clinically actionable data” that should influence a woman’s decision.

Dr. Goss, director of breast cancer research at Massachusetts General Hospital, said the changes in bone structure detected by the new CT technique had not been proved to raise the risk of fractures. In the overall study, the exemestane users did not have more fractures.

And women could take osteoporosis drugs to counter any effect of exemestane, he said.

It is not clear how widely exemestane is being used for prevention. Figures from IMS Health, which tracks drug sales, do not show a significant increase in prescriptions since June, when the original study was published in The New England Journal of Medicine and presented at the annual meeting of the American Society of Clinical Oncology.

Even then, many doctors and patient advocate expressed skepticism that women would use exemestane for prevention. They cited other side effects, like joint pain, and the fact that the drug had not been approved for prevention by the Food and Drug Administration.

A version of this article appears in print on February 7, 2012, on Page D7 of the New York edition with the headline: Cancer-Deterring Drug Found to Harm Bones. Order Reprints|Today's Paper|Subscribe